[ticket/13073] Correctly generate routes from subfolders like /adm

Ticket: https://tracker.phpbb.com/browse/PHPBB3-13073
@Nicofuma your Gist has some issues still. First, where you had `if (!empty($this->config['enable_mod_rewrite']))` I think you meant `if (empty($this->config['enable_mod_rewrite']))`

Also, the url's end up with 2 slashes: `http://localhost/phpbb//page/test`